Slip control apparatus for motor vehicle lock-up clutch

ABSTRACT

An apparatus including a slip control device for controlling an amount of slip of a lock-up clutch disposed between pump and turbine impellers in a power transmission system of a motor vehicle, such that an actual slip speed of the lock-up clutch coincides with a target slip speed, the apparatus comprising a hunting detecting device for determining the presence of a hunting defect if an amplitude of variation of the engine speed is larger than a predetermined first reference while an amplitude of variation of the turbine speed is not larger than a predetermined second reference.

This is a division of application Ser. No. 08/503,744 filed on Jul. 18,1995, now U.S. Pat. No. 5,613,583.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus for controlling the amountof slip of a lock-up clutch provided in a power transmitting system of amotor vehicle.

2. Discussion of the Related Art

In a motor vehicle having a fluid-filled power transmitting deviceequipped with a lock-up clutch, such as a torque converter or fluidcoupling incorporating such a lock-up clutch, it is proposed to controlthe lock-up clutch in a slip control mode (partially slipping mode) suchthat an actual amount of slip (slip speed) of the lock-up clutch,namely, a difference between the speeds of a pump impeller and a turbineimpeller eventually coincides with a predetermined target slip speed,for the purpose of improving the fuel economy of the vehicle whileminimizing the power loss due to slipping of the lock-up clutch. theslip control mode is established when the running condition of thevehicle is in a predetermined slip control area which is intermediatebetween a fully releasing area in which the lock-up clutch should beheld in a fully released state, and a fully engaging area in which thelock-up clutch should be held in a fully engaged state. These fullyreleasing, fully engaging and slip control areas are defined by suitableparameters (e.g., throttle valve opening and vehicle running speed)indicative of the vehicle running condition.

Generally, a lock-up clutch whose slip speed or amount is adjustable isprovided with a piston which is operated by a hydraulic pressure sourcethat permits full engagement of the lock-up clutch. Described in detail,the piston is moved depending upon a difference between hydraulicpressures in two oil chambers, which are formed on the opposite sides ofthe piston. The amount of slip of the lock-up clutch is controlled bycontrolling the pressure difference of the two oil chambers to therebychange a thrust force acting on the piston and the resulting frictionforce of the clutch. Since the hydraulic pressure source that permitsthe full engagement of the lock-up clutch is utilized to control theclutch in the slip control mode, even a small amount of change in thepressure difference of the two oil chambers will result in aconsiderable amount of change in the slip amount of the lock-up clutch.That is, the slip amount of the clutch controlled in a feedback fashiontends to be excessively sensitive to a change in a slip control signalor output generated by a feedback control system. In view of thisdrawback, the slip control output for controlling the amount of slip ofthe lock-up clutch is according to a control equation which includes afeed-forward control value, a feedback control value and a learningcontrol value.

A slip control apparatus as described above includes slip control meansfor controlling the amount of slip of the lock-up clutch in the slipcontrol mode, diagnosing means for detecting a defect, abnormality orfailure associated with the slip control of the lock-up clutch by theslip control means, and slip control terminating means for commandingthe slip control means to terminate the slip control of the clutch whenany defect associated with the slip control is detected by thediagnosing means. For instance, it is proposed to terminate the slipcontrol of the lock-up clutch and place the clutch in the fully releasedstate if the slip speed of a torque converter (difference between theinput shaft speed and the output shaft speed of the torque converter) isheld outside a permissible or normal range for more than a predeterminedlength of time. An example of such slip control apparatus is disclosedin JP-B-62-7430.

In the conventional slip control apparatus described above, however, theslip control of the lock-up clutch is not terminated if the slip speedof the torque converter changes at random or vibrates along a wavinesscurve such that the slip speed instantaneously exceeds the predeterminedthreshold value. In such situation, too, the slip control of the lock-upclutch is considered defective, even though the time period during whichthe slip speed of the torque converter is kept outside the normal rangedoes not exceed the predetermined length of time. In other words, thediagnosing means does not permit highly reliable detection of a defect,abnormality or failure associated with the slip control.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide anapparatus which includes slip control means for controlling the amountof slip of a lock-up clutch in a power transmitting device of a motorvehicle, and which permits highly reliable detection of a defect,abnormality or failure associated with the slip control of the lock-upclutch by the slip control means.

The above object may be achieved according to a first aspect of thepresent invention, which provides an apparatus including slip controlmeans for controlling an amount of slip of a lock-up clutch disposedbetween a pump impeller and a turbine impeller in a power transmissionsystem of a motor vehicle, such that an actual slip speed of the lock-upclutch coincides with a target slip speed, the apparatus comprising: (a)slip-parameter monitoring means for detecting a selected slip-relatedparameter at a predetermined interval while the slip control means is inoperation, the slip-related parameter being selected from the groupconsisting of the actual slip speed of the lock-up clutch and a controlerror between the actual and target slip speeds of the lock-up clutch;(b) summing means for obtaining a sum of values of the selectedslip-related parameter detected by the slip-parameter monitoring means;and (c) defect detecting means for determining the presence or absenceof a defect in slip control of the lock-up clutch by the slip controlmeans, on the basis of the sum as compared with a predeterminedreference.

In the slip control apparatus constructed according to the first aspectof this invention as described above, a suitable slip-related parametersuch as the actual slip speed of the lock-up clutch or a control errorbetween the actual and target slip speeds is detected by theslip-parameter monitoring means at a predetermined interval during slipcontrol of the lock-up clutch by the slip control means. In themeantime, a sum of values of the slip=related parameter detected at thepredetermined interval for a predetermined period of time is obtained bythe summing means. On the basis of the sum as compared with apredetermined reference, the presence or absence of a defect associatedwith the slip control of the clutch is determined by the defectdetecting means.

In the present apparatus wherein the defect in the slip control of thelock-up clutch by the slip control means is detected on the basis of thesum of the actual slip speed values or control error values which aredetected from time to time, the defect can be detected even when theslip speed changes at random or vibrates along a waviness curve, thatis, even where the time period during which the slip speed is heldoutside a permissible or normal range does not exceed a predeterminedtime.

In one preferred form of the apparatus, the slip-parameter monitoringmeans detects the actual slip speed of the lock-up clutch and thesumming means obtains a sum of values of the actual slip speed detectedby the slip-parameter monitoring means. In this case, the defectdetecting means determines the presence of an excessive amount ofengagement of the lock-up clutch as the defect if the sum of values ofthe actual slip speed is smaller than a predetermined reference. Theactual slip speed of the lock-up clutch may be replaced by the controlerror.

In another preferred form of the apparatus, the slip-parametermonitoring means detects the control error and the summing means obtainsa sum of values of the control error detected by the slip-parametermonitoring means. In this case, the defect detecting means determinesthe presence of an excessive release of the lock-up clutch as the defectif the sum of values of the control error is larger than a predeterminedreference. The control error may be replaced by the actual slip speed ofthe lock-up clutch.

It will be obvious that the apparatus may be adapted such that theslip-parameter monitoring means detects both the actual slip speed ofthe lock-up clutch and the control error, while the summing meansobtains both the sum of the actual slip speed values and the sum of thecontrol error values, and the defect detecting means determines thepresence of the excessive engagement defect of the slip control of theclutch if the sum of the actual slip speed values is smaller than thepredetermined reference, and the presence of the excessive releasedefect of the slip control if the sum of the control error values islarger than the predetermined reference.

In a still further preferred form of the invention, the apparatusfurther comprises enabling means (d) for enabling the defect detectingmeans to operate if a slip control output of the slip control meanswhich determines the amount of slip of the lock-up clutch falls within apredetermined range. This arrangement assures improved reliability ofdetermination by the defect detecting means, since the defect detectingmeans is permitted to operate only when the slip control output of theslip control means is held within the predetermined range.

The object indicated above may also be achieved according to a secondaspect of this invention, which provides an apparatus including slipcontrol means for controlling an amount of slip of a lock-up clutchdisposed between a pump impeller and a turbine impeller in a powertransmission system of a motor vehicle, such that an actual slip speedof the lock-up clutch coincides with a target slip speed, the apparatuscomprising: (e) speed-parameter monitoring means for detecting aselected speed-related parameter which varies due to hunting in slipcontrol of the lock-up clutch by the slip control means, while the slipcontrol means is in operation; (f) smoothing means for smoothing thespeed-related parameter detected by the speed-parameter monitoringmeans, to obtain a smoothed speed-related value; and (g) huntingdetecting means for determining the presence or absence of a huntingdefect in slip control of the lock-up clutch by the slip control means,on the basis of a difference of the speed-related parameter from thesmoothed speed-related value.

In the slip control apparatus constructed according to the second aspectof the invention as described above, the speed-related parameter whichvaries due to hunting in the slip control of the lock-up clutch by theslip control means is detected by the speed-parameter monitoring meanswhile the slip control means is in operation. The detected speed-relatedparameter is smoothed to the smoothed speed-related value. On the basisof a difference of the speed-related parameter from the smoothedspeed-related value, the presence or absence of the hunting defect isdetermined by the hunting detecting means.

According to this aspect of the invention wherein the hunting detectingmeans operates on the basis of the difference or variation of thespeed-related parameter with respect to the smoothed value, the huntingdefect may be detected even when the actual slip speed vibrates or evenwhere the time period during which the actual slip speed is held outsidethe normal range is shorter than the predetermined time.

The speed-related parameter which is detected by the speed-parametermonitoring means may be selected from the group consisting of: a speedof an engine of the vehicle connected to the pump impeller; the actualslip speed of the lock-up clutch; and a control error between the actualand target slip speeds of the lock-up clutch.

The object indicated above may also be achieved according to a thirdaspect of this invention, which provides an apparatus including slipcontrol means for controlling an amount of slip of a lock-up clutchdisposed between a pump impeller and a turbine impeller in a powertransmission system of a motor vehicle, such that an actual slip speedof the lock-up clutch coincides with a target slip speed, the apparatuscomprising: (h) engine speed variation determining means for determiningwhether an amplitude of variation of a speed of an engine of the vehicleconnected to the pump impeller is larger than a predetermined firstreference; (i) turbine speed variation determining means for determiningwhether an amplitude of variation of a speed of the turbine impeller islarger than a predetermined second reference; and (j) hunting detectingmeans for determining the presence of a hunting defect in slip controlof the lock-up clutch by the slip control means, if the engine speedvariation determining means determines that the amplitude of variationof the speed of the engine is larger than the predetermined firstreference while the turbine speed variation determining means determinesthat the amplitude of variation of the speed of the turbine impeller isnot larger than the predetermined second reference.

In the slip control apparatus constructed according to the third aspectof this invention, the hunting detecting means determines the presenceof a hunting defect in the slip control of the lock-up clutch, if it isdetermined by the engine speed variation determining means that theamplitude of variation of the engine speed is larger than thepredetermined first reference while it is determined by the turbinespeed variation determining means that the amplitude of variation of theturbine impeller speed is not larger than the predetermined secondreference.

According to this aspect of the invention wherein the hunting detectingmeans operates on the basis of the engine speed variation and theturbine speed variation, the hunting defect may be detected even whenthe actual slip speed vibrates or even where the time period duringwhich the actual slip speed is held outside the normal range does notexceed the predetermined time.

In one preferred form of the apparatus according to the third aspect ofthe invention, wherein the turbine speed variation determining meanscomprises turbine speed obtaining means for obtaining the speed of theturbine impeller, slip speed obtaining means for obtaining the actualslip speed of the lock-up clutch, and variation determining means fordetermining whether an amplitude of variation of a sum of the speed ofthe turbine impeller and the actual slip speed is larger than apredetermined value. In this case, the hunting detecting meansdetermines the presence of the hunting defect if the engine speedvariation is excessive while the sum of the turbine impeller speed andthe actual slip speed of the clutch is not excessive. This arrangementis effective to prevent erroneous determination of the hunting detect,which would be made due to a change of the target slip speed dependingupon the running condition of the vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and optional objects, features, advantages and technicalsignificance of the present invention will be better understood byreading the following detailed description of presently preferredembodiments of the invention, when considered in conjunction of theaccompanying drawings, in which:

FIG. 1 is a schematic view illustrating a part of a power transmittingsystem of a motor vehicle, which incorporates a torque converter havinga lock-up clutch to be controlled by a slip control apparatus accordingto the present invention;

FIG. 2 is a table indicating a relationship between the operatingpositions of an automatic transmission connected to the torque converterand the respective combinations of the operating states of first andsecond solenoid-operated valves of the slip control apparatus;

FIG. 3 is a block diagram showing a control system for the motorvehicle, which includes a transmission controller incorporating the slipcontrol apparatus for the lock-up clutch;

FIG. 4 is a view illustrating a part of a hydraulic control device shownin FIG. 3, which incorporates a circuit for controlling the lock-upclutch;

FIG. 5 is a graph indicating an output characteristic of a linearsolenoid valve provided in the lock-up clutch control circuit of FIG. 4;

FIG. 6 is a graph indicating an output characteristic of a lock-upclutch control valve provided in the lock-up clutch control circuit ofFIG. 4, namely, a relationship between a pilot pressure P_(SLU) receivedby the lock-up clutch control valve and a pressure difference ΔP ofengaging and releasing oil chambers of the lock-up clutch;

FIG. 7 is a view indicating boundaries defining different control areasof the lock-up clutch in relation to the running condition of thevehicle, which boundaries are stored in the slip control apparatus shownin FIG. 3;

FIGS. 8A-8D are block diagrams illustrating the functions of variousfunctional means incorporated in the slip control apparatus;

FIG. 9 is a graph indicating a relationship used to determine targetslip speed TN_(SLP) used for slip control of the lock-up clutch;

FIG. 10 is a flow chart illustrating a diagnostic routine for diagnosingthe slip control of the lock-up clutch according to a first embodimentof the invention of FIG. 8B;

FIG. 11 is a graph indicating an overshoot of engine speed N_(E) uponchanging of the target slip speed TN_(SLP), which may cause erroneousdetection of a defect associated with the slip control of the lock-upclutch;

FIG. 12 is a flow chart illustrating a diagnostic routine for diagnosingthe slip control of the lock-up clutch according to a second embodimentof this invention of FIG. 8C;

FIG. 13 is a graph indicating a variation of control error DN_(SLP) withrespect to smoothed control error value DN_(SLPSM) ;

FIG. 14 is a flow chart illustrating a diagnostic routine for diagnosingthe slip control of the lock-up clutch according to a third embodimentof the invention of FIG. 8D; and

FIG. 15 is a time chart indicating hunting of the engine speed N_(E)which is detected in the embodiment of FIG. 14.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to the schematic view of FIG. 1, there is shown a partof a power transmitting system of a motor vehicle, wherein powergenerated by an engine 10 is transmitted to a differential gear deviceand drive wheels through a torque converter 12 equipped with a lock-upclutch 32, and an automatic transmission 14 which includes threeplanetary gear sets to selectively establish a plurality of operatingpositions (gear positions).

The torque converter 12 includes; a pump impeller 18 connected to acrankshaft 16 of the engine 10; a turbine impeller 22 fixed to an inputshaft of the automatic transmission 14 and rotatable by the pumpimpeller 18; a stator impeller 28 fixed to a stationary member in theform of a housing 26 through a one-way clutch 24; and theabove-indicated lock-up clutch 32 connected to the input shaft 20through the turbine impeller 22. The pump impeller 18 includes aradially outer portion which is U-shaped in cross section, and aplurality of curved vanes which are arranged in the circumferentialdirection and formed so as to cause a flow of a working oil, which flowincludes a component moving toward the turbine impeller 22 on the sideof the engine 10. The turbine impeller 22 includes a plurality of curvedvanes opposed to the vanes of the pump impeller 18. In operation of thetorque converter 12, the turbine impeller 22 is rotated by the oil flowfrom the vanes of the pump impeller 18 rotated by the engine 10. Thelock-up clutch 32 includes a piston 30 which engages a hub of theturbine impeller 22 such that the piston 30 is axially slidable relativeto and rotatable with the turbine impeller 22.

The piston 30 of the lock-up clutch 32 divides an interior of the torqueconverter 12 into two oil chambers 33 and 35. The lock-up clutch 32 isreleased and engaged by axial movements of the piston 32 depending upona difference between oil pressures in these two oil chambers 33, 35,which will be hereinafter referred to as a releasing oil chamber 33 andan engaging oil chamber 35, respectively. Described more specifically,the piston 30 is retracted to its fully retracted position when thepressure in the releasing oil chamber 33 is increased while the engagingoil chamber 35 is drained. When the pressure in the engaging oil chamber35 is increased while the releasing oil chamber 33 is held at the lowestlevel, the piston 30 is advanced to its fully advanced position. In thefully retracted position of the piston 30, the lock-up clutch 32 isplaced in its fully released position in which the torque received bythe pump impeller 18 is amplified or boosted at a ratio depending uponthe ratio of the input and output speeds of the torque converter 12. Inthe fully advanced position of the piston 30, the lock-up clutch 32 isplaced in the fully engaged position in which the frictional couplingportion of the clutch 32 is forced against the radially outer U-shapedportion of the pump impeller 18, whereby the pump impeller 18 isdirectly connected to the input shaft 20, that is, the crankshaft 16 asan input member of the torque converter 12 is directly connected to theinput shaft 20 of the transmission 14, which serves as an output memberof the torque converter 12. When the pressure in the releasing oilchamber 33 is increased to a predetermined level while the pressure inthe engaging oil chamber 35 is held at a higher level, the piston 30 isadvanced to a predetermined position in which the frictional couplingportion of the lock-up clutch is located near the corresponding couplingportion (radially outer U-shaped portion) of the pump impeller 18. Thepredetermined level of the pressure in the releasing oil chamber 33indicated above is determined by a second term ("feed forward term") ofa right member of an equation (2) which will be described.

The automatic transmission 14 includes: the input shaft 20, a first, asecond and a third planetary gear set 34, 36, 38; an output gear 39which rotates with a ring gear of the third planetary gear set 38; andan output shaft in the form of a counter shaft 40 which connects theoutput gear 39 and the differential gear device. The planetary gear sets34, 36, 38 include components which are connected integrally with eachother, and components which are connected to each other when threeclutches C0, C1, C2 are selectively engaged. The planetary gear sets 34,36, 38 also include components which are fixed or connected to thehousing 26 and thereby inhibited from rotating when four brakes B0, B1,B2, B3 are selectively engaged. The planetary gear sets 34, 36, 38further include components which are connected to each other or to thehousing 26 through three one-way clutches F0, F1, F2, depending upon therotating directions of the components.

Each of the clutches C0, C1, C2 and brakes B0, B1, B2, B3 may consist ofa multiple-disk clutch, or a band brake which uses two bands wound inopposite directions. These clutches and brakes are operated byrespective hydraulically operated actuators, which are controlled by anelectronic transmission controller 184 shown in FIG. 3 (which will bedescribed), so as to selectively establish a plurality of operatingpositions of the automatic transmission 14. That is, the automatictransmission 14 has four forward drive positions, first-speed ("1st"),second-speed ("2nd"), third-speed ("3rd") and overdrive ("O/D")positions, and one backward drive position "R", as indicated in FIG. 2.The four forward drive positions "1st", "2nd", "3rd" and "O/D" haverespective different speed ratios I which decrease in the order ofdescription. The speed ratio I is defined as the speed of the inputshaft 20 divided by the speed of the counter shaft (output shaft) 40.

It is to be noted that the lower halves of the torque converter 12 andautomatic transmission 14 and the upper half of the counter shaft 40 arenot shown in FIG. 1 in the interest of simplification, since theseelements 12, 14, 40 are symmetrical with respect to their axes ofrotation.

Referring next to the block diagram of FIG. 3, there will be described acontrol system provided to control the engine 10, lock-up clutch 32 andautomatic transmission 14 of the motor vehicle. The control systemincludes the electronic transmission controller 184 indicated above,which is adapted to control a hydraulic control device 44. The hydrauliccontrol device 44 includes a transmission control circuit for shiftingthe automatic transmission 14 to an appropriate one of the operatingpositions, and a lock-up clutch control circuit for controlling theoperating state of the lock-up clutch 32. The transmission controlcircuit is provided with a first and a second solenoid-operated valveS1, S2, which have respective solenoid coils. The clutches C0, C1, C2and brakes B0, B1, B2, B3 are selectively engaged to selectivelyestablish the operating positions ("1st", "2nd", "3rd" and "O/D") of thetransmission 14, depending upon respective combinations of the operatingstates of the first and second solenoid-operated valves S1, S2, asindicated in FIG. 2. In this figure, "o" indicates the energization ofthe solenoid coils of the valves S1, S2 or the engagement of theclutches and brakes.

The lock-up clutch control circuit of the hydraulic control device 44includes a third solenoid-operated valve S3, a lock-up relay valve 52, alinear solenoid valve SLU, and a lock-up clutch control valve 56, asshown in FIG. 4. The third solenoid-operated valve S3 has a solenoidcoil 48 which is turned on and off. When the coil 48 is on, the valve 53generates a LOCK-UP SWITCHING pressure P_(SW). The lock-up relay valve52 has a releasing state and an engaging state for releasing andengaging the lock-up clutch 32, respectively, depending upon whether thepilot pressure P_(SW) is generated by the valve S3. The linear solenoidvalve SLU is adapted to generate a SLIP CONTROL pilot pressure P_(SLU)corresponding to a SLIP CONTROL current I_(SLU) supplied from thetransmission controller 184. The lock-up clutch control valve 56 isadapted to regulate a pressure difference ΔP between the pressures inthe engaging and releasing oil chambers 35, 33 of the torque converter12, according to the SLIP CONTROL pilot pressure P_(SLU) received fromthe linear solenoid valve SLU, for thereby controlling an amount of slipof the lock-up clutch 32.

As shown in FIG. 4, the hydraulic control device 44 includes a pump 60for pressuring a working oil sucked from a suitable reservoir through astrainer 58. The pump 60 is driven by the engine 10. The pressure of theoil delivered by the pump 60 is adjusted to a first line pressure Pl1 bya first pressure regulating valve 62 of an overflow type. The firstpressure regulating valve 62 is arranged to receive a THROTTLE pilotpressure indicative of an opening TAP of a first throttle valve 166(FIG. 3), and regulate the first line pressure Pl1 in a first pressureline 64 such that the pressure Pl1 increases with the THROTTLE pilotpressure. The hydraulic control device 44 further has a second pressureregulating valve 66 of an overflow type, which is adapted to regulatethe pressure of the oil received from the first pressure regulatingvalve 62, to a second line pressure Pl2 according to the THROTTLEpressure, so that the second line pressure Pl² corresponds to the outputtorque of the engine 10. The device 44 further has a third pressureregulating valve 68, which is a pressure reducing valve adapted toreduce the first line pressure Pl1 to a predetermined third linepressure Pl3.

The motor vehicle has a shift lever 174 (FIG. 3) which has six operatingpositions "P" (PARKING), "R" (REVERSE), "N" (NEUTRAL), "D" (DRIVE), "S"(SECOND) and "L" (LOW), as indicated in FIG. 2. The hydraulic controldevice 44 includes a manual valve 70 (FIG. 4) adapted to generate aREVERSE pressure P_(R) when the shift lever 174 is placed in the REVERSEposition "R" (which is the backward drive position referred to abovewith respect to the automatic transmission 14). The device 44 alsoincludes an OR valve 72 which is adapted to generate a higher one of aBRAKE B2 pressure P_(B2) and the REVERSE pressure P_(R), which serves asthe LOCK-UP SWITCHING pilot pressure P_(SW) generated when the valve S3is turned ON as explained below in detail. The BRAKE B2 pressure P_(B2)is generated to engage the brake B2 for establishing the second-speed("2nd"), third-speed ("3rd") and overdrive ("O/D") positions.

The lock-up relay valve 52 has: a releasing port 80 communicating withthe releasing oil chamber 33; an engaging port 82 communicating with theengaging oil chamber 35; an input port 84 adapted to receive the secondline pressure Pl2; a first drain port 86 through which the oil in theengaging oil chamber 35 is discharged when the lock-up clutch 32 isreleased; a second drain port 88 through which the oil in the releasingoil chamber 33 is discharged when the lock-up clutch 32 is engaged; asupply port 90 adapted to receive the oil discharged from the secondpressure regulating valve 66 so that the oil is cooled during engagementof the lock-up clutch 32; a spool 92 operable between an ON position andan OFF position, for switching the mutual communication or connection ofthe ports indicated above; a spring 94 for biasing the spool 92 towardthe OFF position; a plunger 96 abuttable on the end of the spool 92 onthe side of the spring 94; an oil chamber 98 defined between theabove-indicated end of the spool 92 and the opposed end of the plunger96, and adapted to receive the REVERSE pressure P_(R) from the manualvalve 70; an oil chamber 100 partially defined by the other end of theplunger 96 and adapted to receive the first line pressure Pl1; and anoil chamber 102 partially defined by the other end of the spool 92 andadapted to receive the LOCK-UP SWITCHING pressure P_(SW) from the thirdsolenoid-operated valve S3, for generating a thrust force for moving thespool 92 toward the ON position.

The third solenoid-operated valve S3 has a ball which is seated on avalve seat to disconnect a line communicating with the oil chamber 102of the lock-up relay valve 52 and the OR valve 72 when the solenoid coil48 is de-energized or OFF. In this state, the LOCK-UP SWITCHING pilotpressure P_(SW) is not applied to the oil chamber 102. When the coil 48is energized or ON, the ball is unseated to permit the communicationbetween the OR valve 72 and the oil chamber 102, whereby the LOCK-UPSWITCHING pressure P_(SW) is applied to the oil chamber 102. In the OFFstate of the valve S3, therefore, the spool 92 of the lock-up relayvalve 52 is moved to its OFF position by the biasing force of the spring94 and a force based on the first line pressure Pl1 in the oil chamber100, whereby the input port 84 communicates with the releasing port 80while the first drain port 86 communicates with the engaging port 82. Asa result, a pressure Poff in the releasing oil chamber 33 is made higherthan a pressure Pon in the engaging oil chamber 35, to thereby releasethe lock-up clutch 32, while at the same time the engaging chamber 35 isdrained through the first drain port 86, an oil cooler 104 and a checkvalve 106.

In the ON state of the valve S3, on the other hand, the LOCK-UPSWITCHING pilot pressure P_(SW) is applied to the oil chamber 102, andthe spool 92 is moved by a force based on the pressure P_(SW) againstthe biasing force of the spring 94 and the force based on the first linepressure Pl1 in the oil chamber 100, whereby the input port 84communicates with the engaging port 82 while the first and second drainports 86, 88 communicate with the supply and releasing ports 90, 80,respectively. As a result, the pressure Pon in the engaging oil chamber35 is made higher than the pressure Poff in the releasing oil chamber33, to thereby engage the lock-up clutch 32, while at the same time thereleasing oil chamber 33 is drained through the second drain port 88 andthe lock-up clutch control valve 56.

The linear solenoid valve SLU is a reducing valve adapted to reduce thepredetermined third line pressure Pl3 to the SLIP CONTROL pilot pressureP_(SLU), such that the pilot pressure P_(SLU) increases with an increasein the SLIP CONTROL current I_(SLU) supplied from the transmissioncontroller 184, namely, increases with an increase in a duty ratioD_(SLU) of the linear solenoid valve SLU. The thus controlled pilotpressure P_(SLU) is applied to the lock-up clutch control valve 56. Thelinear solenoid valve SLU has: a supply port 110 adapted to receive thethird line pressure Pl3; an output port 112 from which the SLIP CONTROLpilot pressure P_(SLU) is applied to the valve 56; a spool 114 forclosing and opening the ports 110, 112; a spring 115 for biasing thespool 114 in a valve closing direction; a spring 116 for biasing thespool 114 in a valve opening direction by a force smaller than that ofthe spring 115; a Solenoid coil 118 for biasing the spool 114 in thevalve opening direction by a force determined by the SLIP CONTROLcurrent I_(SLU) ; and an oil chamber 120 adapted to receive a feedbackpressure (SLIP CONTROL pilot pressure P_(SLU)) which biases the spool114 in the valve closing direction. The spool 114 is moved to a positionof equilibrium between a sum of the biasing forces of the solenoid coil118 and the spring 116 and a sum of the biasing force of the spring 115and a force based on the feedback pressure P_(SLU).

The lock-up clutch control valve 56 has: a line pressure port 130adapted to receive the second line pressure Pl2; an input port 132adapted to receive the oil discharged from the releasing oil chamber 33through second drain port 88 of the valve 52; a drain port 134 throughwhich the oil received by the input port 132 is discharged; a spool 136operable between a first position (indicated at left in FIG. 4) and asecond position (indicated at right in FIG. 4); a plunger 138 abuttableon the spool 136 for biasing the spool 136 toward the first position; anoil chamber 140 adapted to receive the SLIP CONTROL pilot pressureP_(SLU) for biasing the plunger 138 so as to generate a thrust forcewhich biases the spool 136 toward the first position; an oil chamber 142adapted to receive the oil pressure Poff in the releasing oil chamber33, for biasing the plunger 138 so as to generate a thrust force whichbiases the spool 136 toward the first position; an oil chamber 144adapted to receive the oil pressure Pon in the engaging oil chamber 35,for generating a thrust force for biasing the spool 136 toward thesecond position; and a spring 146 received in the oil chamber 144, forbiasing the spool 136 toward the second position.

In the first position of the spool 136 of the lock-up clutch controlvalve 56, the input port 132 communicates with the drain port 134 tocause the releasing oil chamber 33 to be drained, for thereby increasingthe pressure difference ΔP (=Pon-Poff) of the oil chambers 33, 35. Inthe second position of the spool 136, the input port 132 communicateswith the line pressure port 130 to cause the second line pressure Pl2 tobe applied to the releasing oil chamber 33, for thereby reducing thepressure difference ΔP.

The plunger 138 has a first land 148 adjacent to the oil chamber 142,and a second land 150 remote from the oil chamber 142. The first land148 has a cross sectional area A1, and the second land 150 has a crosssectional area A2 larger than the area A1. The spool 136 has a thirdland 152 adjacent to the pilot pressure oil chamber 140, and a fourthland 154 remote from the oil chamber 140. The third land 152 has a crosssectional area A3, and the fourth land 154 has a cross sectional areaequal to the cross sectional area A1. In this arrangement of the lock-upclutch control valve 56, the plunger 138 and the spool 136 are movedtogether as a unit with the plunger 138 held in abutting contact withthe spool 136. With the movement of the plunger and spool 138, 136, thepressure difference ΔP (=Pon-Poff) on the opposite sides of the piston30 of the lock-up clutch 32 is controlled depending upon the SLIPCONTROL pilot pressure P_(SLU) generated by the linear solenoid valveSLU. The pressure difference ΔP changes with the pilot pressure P_(SLU)as shown in FIG. 6, at a rate or gradient represented by a value(A2-A1)/A1 included in the following equation (1):

    ΔP=Pon-Poff= (A2-A1)/A1!P.sub.SLU -Fs/A1             (1)

where, Fs: biasing force of the spring 146.

The graph of FIG. 6 indicates the output characteristic of the lock-upclutch control valve 56, namely, the relationship between the pressuredifference ΔP generated by the valve 56 and the SLIP CONTROL pilotpressure P_(SLU) generated by the valve SLU. While the lock-up clutchcontrol valve 56 is ON with the spool 136 placed in the first position,an increase in the pilot pressure P_(SLU) results in an increase in thepressure difference ΔP of the engaging and releasing oil chambers 35,33, and thereby causes a decrease in a slip speed N_(SLP) of the lock-upclutch 32, while a decrease in the pilot pressure P_(SLU) causes anincrease in the slip speed N_(SLP). The slip speed N_(SLP) is adifference (N_(P) -N_(T)) between a speed N_(P) of the pump impeller 18(speed N_(E) of the engine 10) and a speed N_(T) of the turbine impeller22 (speed Nin of the input shaft 20).

Referring back to the block diagram of FIG. 3, the motor vehicle hasvarious sensors and switches including: an engine speed sensor 160 fordetecting the speed N_(E) of the engine 10, that is, speed N_(P) of thepump impeller 18; an intake air quantity sensor 162 for detecting aquantity Q of an intake air sucked into the engine 10 through an intakepipe; an intake air temperature sensor 164 for detecting a temperatureT_(AIR) of the intake air; a throttle sensor 167 for detecting theopening TAP of the first throttle valve 166 operated by an acceleratorpedal 165, the throttle sensor 167 being equipped with an idlingposition switch for detecting the idling position of the throttle valve166; a vehicle speed sensor 168 for detecting a running speed V of thevehicle on the basis of a speed Nout of the output shaft 40 of theautomatic transmission 40; a water temperature sensor 170 for detectinga temperature T_(WA) of a coolant water of the engine 10; a brake switch172 for detecting an operation of a brake pedal; a shift position sensor176 for detecting a currently selected operating position Ps of theautomatic transmission 40, namely, a currently selected one of theoperating positions "L", "S", "D", "N", "R" and "P" of the shift lever174; a turbine speed sensor 178 for detecting the speed N_(T) of theturbine impeller 22, that is, the speed Nin of the input shaft 20 of thetransmission 20; and an oil temperature sensor 180 for detecting atemperature T_(OIL) of the working oil in the hydraulic control device44. The output signals generated by the above sensors and switch areapplied directly or indirectly to an electronic engine controller 182and the electronic transmission controller 184. The two controllers 182,184 are connected to each other by a communication interface, forapplying the necessary signals to each other.

The transmission controller 184 is comprised of a so-calledmicrocomputer incorporating a central processing unit (CPU), a read-onlymemory (ROM), a random-access memory (RAM) and an interface. The CPUprocesses the input signals according to various control programs storedin the ROM, while utilizing a temporary data storage function of theRAM, for controlling the automatic transmission 14 and the lock-upclutch 32 by controlling the first, second and third solenoid-operatedvalves S1, S2, S3 and the linear solenoid valve SLU.

For controlling the automatic transmission 14 so as to shift thetransmission 14 to the appropriate operating position, a plurality ofshift patterns are stored in the ROM, and one of the shift patternswhich corresponds to the currently selected position of the transmission14 is selected to determine the operating position (one of the fourforward drive positions) to which the transmission 14 should be shifteddown or up. For instance, each shift pattern consists of a shift-downboundary line and a shift-up boundary line which are relationshipsbetween the throttle valve opening TAP and the vehicle speed V. On thebasis of the determined forward drive position to which the transmission14 should be shifted, the solenoid-operated valves S1 and S2 aresuitably controlled (with their solenoid coils being suitably energizedor de-energized), so as to establish an appropriate combination of theoperating states of the clutches and brakes C0, C1, C2, B0, B1, B2, B3,which combination corresponds to the determined forward drive position.

The transmission controller 184 is adapted to control the lock-up clutch32 in the manner explained below, when the vehicle is running with thetransmission 14 placed in the third-speed or fourth-speed or overdriveposition ("3rd" or "O/D"), for example. For controlling the lock-upclutch 32 differently depending upon the running condition of thevehicle, predetermined boundaries defining three different control areasas indicated in FIG. 7 are stored in the ROM. For instance, theboundaries are relationships between the throttle valve opening TAP andthe output speed Nout of the output shaft 40 of the transmission 14(vehicle speed V). Described more specifically, these boundaries definea fully releasing area in which the lock-up clutch 32 should be fullyreleased, a fully engaging area in which the clutch 32 should be fullyengaged, and a slip control area in which the amount of slip of theclutch 32 should be suitably controlled according to the principle ofthe present invention as described below in detail. Depending upon thecurrently detected throttle opening TAP and output speed Nout, one ofthe three control areas is determined or selected by the CPU of thetransmission controller 184, according to the boundaries stored in theROM.

When the vehicle running condition (TAP and Nout) is in the slip controlarea, the lock-up clutch 32 is controlled to be held in a partiallyslipping state for transmitting power of the engine 10 to the automatictransmission 14 so as to maximize the fuel economy of the vehicle whileabsorbing a torque variation of the engine 10 to assure high drivabilityof the vehicle. The determination as to whether the vehicle runningcondition falls in the slip control area according to the boundaries ofFIG. 7 stored in the ROM is effected while the vehicle is accelerating.In this respect, it is noted that the amount of slip of the lock-upclutch 32 is also controlled while the vehicle is coasting ordecelerating with the throttle valve 166 placed in the idling position.This slip control is effected to increase an effect of the fuel-cutcontrol of the engine 10. In this case, however, the slip control areais determined on the basis of only the vehicle speed V, since thethrottle opening TAP is zero during the coasting of the vehicle.

If the CPU of the controller 184 determines that the vehicle runningcondition falls in the fully engaging area, the solenoid coil of thethird solenoid-operated valve S3 is energized to turn ON the lock-uprelay valve 52, and the SLIP CONTROL current I_(SLU) applied to thelinear solenoid valve SLU is reduced to the minimum value, whereby thelock-up clutch 32 is fully engaged. If the vehicle running condition isdetermined to be in the fully releasing area, the solenoid coil of thevalve S3 is de-energized to turn OFF the lock-up relay valve 52, so thatthe lock-up clutch 32 is fully released irrespective of the SLIP CONTROLcurrent I_(SLU). If the vehicle running condition is determined to be inthe slip control area, the solenoid coil of the valve S3 is energized toturn ON the lock-up relay valve 52, and the SLIP CONTROL current I_(SLU)to be applied to the valve SLU, namely, a duty ratio D_(SLU) of thevalve SLU is adjusted according to the following equation (2) to controlthe amount of slip of the lock-up clutch 32 in a slip control mode:

    D.sub.SLU (=I.sub.SLU)=DFWD+KGD+DFB                        (2)

For instance, the duty ratio D_(SLU) is calculated to zero an error ΔE(=N_(SLP) -TNSLP) between a target slip speed TNSLP and the actual slipspeed N_(SLP) (=N_(E) -N_(T)) of the lock-up clutch 32. The first termDFWD of the right member of the above equation (2) is a feed-forwardcontrol value, which varies as a function of the output torque of theengine 10, for example. The second term KGD is a learning control valuewhich changes so as to reflect the varying characteristics of thelock-up clutch 32. The third term DFB is a feedback control valueconsisting of a proportional value, a differential value and an integralvalue of the control error ΔE.

The feedback control value DFB is obtained according to the followingequation (3):

    DFB=K.sub.p  ΔE+(1/T1) ∫ΔEdt+T.sub.D (dΔE/dt)!(3)

The electronic engine controller 182 is comprised of a microcomputersimilar to that of the transmission controller 184, which has a CPUadapted to process the input signals according to programs stored in aROM while utilizing a temporary data storage function of a RAM, forcontrolling the engine 10, more specifically, for effecting a fuelinjection control for controlling a fuel injection valve 186 so as tooptimize the combustion condition of the engine 10, an ignition controlfor controlling an ignitor 188 so as to optimize the ignition timing, atraction control for controlling a second throttle valve 192 via athrottle actuator 190 so as to control the traction force of the vehiclewhile preventing slipping of the drive wheels on the road surface, and afuel-cut control for holding the fuel injection valve 186 closed whilethe engine speed N_(E) is higher than a predetermined fuel-cut thresholdlevel N_(CUT) during coasting of the vehicle, so that the fuel economyof the vehicle is improved.

Referring next to the block diagrams of FIGS. 8A-8D, there will bedescribed the functions of various functional means provided in theelectronic transmission controller 184. The transmission controller 184incorporates slip control means 196, slip-parameter monitoring means198, summing means 200, defect detecting means 202, enabling means 206,speed-parameter monitoring means 208, smoothing means 210, huntingdetecting means 212, engine speed variation determining means 214,turbine speed variation determining means 216, and hunting detectingmeans 218.

When the vehicle running condition is determined to fall in the slipcontrol area explained above by reference to FIG. 7, the slip controlmeans 196 applies the SLIP CONTROL current I_(SLU) to the linearsolenoid valve SLU, to control the amount of slip (slip speed N_(SLP)=N_(E) -N_(T)) of the lock-up clutch 32 such that the detected slipspeed N_(SLP) coincides with the target slip speed TN_(SLP). To thisend, the target slip speed TN_(SLP) is determined on the basis of thethrottle opening TAP and the output shaft speed Nout (corresponding tothe vehicle running speed V), and according to the predeterminedrelationship between these parameters TAP, Nout and the target slipspeed TN_(SLP) as indicated in the graph of FIG. 9, by way of example.The SLIP CONTROL current I_(SLU) corresponds to the duty ratio D_(SLU)of the valve SLU calculated according to the above equation (2). It willbe understood that the SLIP CONTROL current I_(SLU) or the duty ratioD_(SLU) of the linear solenoid valve SLU is considered to be the slipcontrol output of the slip control means 196.

The slip-parameter monitoring means 198 is adapted to detect, as aparameter related to the slip of the lock-up clutch 32, the actual slipspeed N_(SLP) of the lock-up clutch 32 and the control error ΔE whilethe slip control means 196 is in operation. The summing means 200 isadapted to obtain a sum SN_(SLP) of the slip speed values N_(SLP)detected in successive cycles of execution of a diagnostic routine ofFIG. 10 which will be described. The summing means 200 also obtains asum SΔE of the control errors ΔE detected in the successive cycles ofexecution of the diagnostic routine. The detect detecting means 202determines the presence or occurrence of an "excessive engagementdefect" and an "excessive release defect" of the lock-up clutch 32, onthe basis of the obtained sum SN_(SLP) or SΔE as compared with apredetermined reference, as described below in detail by reference tothe flow chart of FIG. 10. The enabling means 206 enables the defectdetecting means 202 to operate while the duty ratio D_(SLU) of thelinear solenoid valve SLU as determined by the slip control means 196 isheld within a predetermined range.

The speed-parameter monitoring means 208 is adapted to detect aspeed-related parameter which varies due to hunting in the slip controlof the lock-up clutch 32 by the slip control means 196. The parameterdetected by the speed-parameter monitoring means 208 may be the enginespeed N_(E), slip speed N_(SLP) or an amount of change of the controlerror ΔE=DN_(SLP). However, the turbine impeller speed N_(T) cannot beused as the parameter, because it does not vary due to the slip controlhunting while it vibrates due to juddering of the engine 10. Thesmoothing means 210 is adapted to smooth the output of thespeed-parameter monitoring means 208. The hunting detecting means 212determines the presence or occurrence of a hunting defect of the slipcontrol means 196, on the basis of time periods CHGP and CHGM duringwhich the amplitude of the output of the speed-parameter monitoringmeans 208 with respect to the smoothed value of the output.

The engine speed variation determining means 214 determines whether theamplitude of the variation of the engine speed N_(E) is larger than apredetermined threshold. The turbine speed variation determining means216 determines whether the amplitude of the variation of the speed N_(T)of the turbine impeller 22 is larger than a predetermined threshold.Preferably, however, the turbine speed variation determining means 216is adapted to determine whether the amplitude of the variation of avalue (N_(T) +TN_(SLP)) is larger than a predetermined threshold. Thehunting detecting means 218 determines the presence or occurrence of ahunting defect of the slip control means 196, if the amplitude of thevariation of the engine speed N_(E) is larger than the threshold whenthe amplitude of the variation of the turbine impeller speed N_(T) orvalue (N_(T) +TN_(SLP)) is not larger than the threshold.

Reference is now made to the flow chart of FIG. 10, which shows thediagnostic routine executed by the transmission controller 184 accordingto a first embodiment of this invention schematically illustrated inFIG. 8B, for diagnosing the slip control of the lock-up clutch 32 todetect the excessive engagement defect and excessive release defect ofthe clutch 32. The routine includes steps SK1-SK7 for effecting a firstdiagnosis to detect the excessive engagement defect of the clutch 32,and steps SK8-SK14 for effecting a second diagnosis to detect theexcessive release of the clutch 32.

The routine of FIG. 10 is initiated with step SK1 to determine whetherfirst diagnostic conditions for initiating the first diagnosis aresatisfied. These first diagnostic conditions include a condition thatthe engagement force of the lock-up clutch 32 determined by the dutyratio D_(SLU) is smaller than a predetermined reference. For example,the engagement force is considered to be smaller than the reference ifthe duty ratio D_(SLU) determined by the SLIP CONTROL current I_(SLU) ishigher than a predetermined reference, for instance, not lower than 80%.If an affirmative decision (YES) is obtained in step SK1, the followingsteps SK2-SK7 are implemented to effect the first diagnosis. Thus, stepSK1 corresponds to the enabling means 206 which enables the defectdetecting means 202 to operate while the duty ratio D_(SLU) is 80% orhigher. In other words, the first diagnosis to detect the excessiveengagement defect of the lock-up clutch 32 is effected only when theengagement force of the clutch 32 as determined by the output of theslip control means 196 is smaller than the predetermined reference.

If a negative decision (NO) is obtained in step SK1, the control flowgoes to step SK7 to reset the sum SN_(SLP) and the content of a timecounter CT1 to "0". Step SK7 is followed by step SK8 which will bedescribed. With the affirmative decision (YES) obtained in step SK1, thecontrol flow goes to step SK2 corresponding to the slip-parametermonitoring means 198 and summing means 200, wherein the actual slipspeed N_(SLP) of the lock-up clutch 32 is calculated, and the currentsum SN_(SLP) i of the slip speeds NSLP is calculated according to thefollowing equation (4):

    SNSLPi=SN.sub.SLPi-1 +NSLP

Step SK2 is followed by step SK3 to determine whether the content of thetime counter CT1 is equal to or larger than a predetermined value T1.Initially, a negative decision (NO) is obtained in step SK3, and stepSK4 is implemented to increment the time counter CT1. Step SK4 isfollowed by step SK8. The predetermined value T1 corresponds to a timeperiod during which the actual slip speed values N_(SLP) detected insuccessive cycles of execution of the routine are summed to obtain thesum SN_(SLP). For example, the predetermined value T1 corresponds toabout three seconds.

When the content of the time counter CT1 has reached the predeterminedvalue T1 during repeated execution of the routine, an affirmativedecision (YES) is obtained in step SK3, and the control flow goes tostep SK5 to determine whether the sum SN_(SLP) is smaller than apredetermined reference SN_(SLPK). This reference SN_(SLPK) is a lowerlimit of the sum SN_(SLP) below which the engagement force of thelock-up clutch 32 is considered to be excessively large where the dutyratio D_(SLU) is not lower than 80% (where the affirmative decision isobtained in step SK1). For example, the reference SN_(SLPK) is set to bein the neighborhood of 10×T1 r.p.m.

If the slip control of the lock-up clutch 32 is normally effected by theslip control means 196, a negative decision (NO) is obtained in stepSK5. In this case, step SK7 is implemented to reset the sum SN_(SLP) andthe time counter CT1. If the sum SN_(SLP) of the successively obtainedslip speed values N_(SLP) is smaller than the lower limit SN_(SLPK), itmeans that the engagement force of the clutch 32 is excessively largerthan the value as attained according to the duty ratio D_(SLU)corresponding to the SLIP CONTROL current I_(SLU) generated by the slipcontrol means 196. In this instance, step SK5 is followed by step SK6 todetermine that there exists the excessive engagement defect of thelock-up clutch 32. This fact is stored in the transmission controller184. Step SK6 is followed by step SK7. It will be understood that stepsSK5 and SK6 correspond to the defect detecting means 202.

As indicated above, step SK7 is followed by step SK8 to determinewhether second diagnostic conditions for initiating the second diagnosisare satisfied. These second diagnostic conditions include a conditionthat the engagement force of the lock-up clutch 32 determined by theduty ratio D_(SLU) is larger than a predetermined reference. Forexample, the engagement force is considered to be larger than thereference if the duty ratio D_(SLU) determined by the SLIP CONTROLcurrent I_(SLU) is lower than a predetermined reference, for instance,not higher than 40%. If an affirmative decision (YES) is obtained instep SK8, the following steps SK9-SK14 are implemented to effect thesecond diagnosis. Thus, step SK8 also corresponds to the enabling means206 which enables the defect detecting means 202 to operate while theduty ratio D_(SLU) is 40% or lower. In other words, the second diagnosisto detect the excessive release of the lock-up clutch 32 is effectedonly when the engagement force of the clutch 32 as determined by theoutput of the slip control means 196 is larger than the predeterminedreference.

If a negative decision (NO) is obtained in step SK8, the control flowgoes to step SK14 to reset the sum SDN_(SLP) and the content of a timecounter CT2 to "0". One cycle of execution of the routine of FIG. 10 iscompleted when step SK14 has been implemented. With the affirmativedecision (YES) obtained in step SK8, the control flow goes to step SK9corresponding to the slip-parameter monitoring means 198 and summingmeans 200, wherein the actual control error DN_(SLP) is calculated, andthe current sum SDN_(SLP) i of the control error values DN_(SLP) iscalculated according to the following equation (5):

    SDN.sub.SLPi =SDN.sub.SLPi-1 +DN.sub.SLP

Step SK9 is followed by step SK10 to determine whether the content ofthe time counter CT2 is equal to or larger than a predetermined valueT2. Initially, a negative decision (NO) is obtained in step SK10, andstep SK11 is implemented to increment the time counter CT2. Step SK11 isfollowed by step SK14. The predetermined value T2 corresponds to a timeperiod during which the control error values DN_(SLP) detected insuccessive cycles of execution of the routine are summed to obtain thesum SDN_(SLP). For example, the predetermined value T2 corresponds toabout three seconds.

When the content of the time counter CT2 has reached the predeterminedvalue T2 during repeated execution of the routine, an affirmativedecision (YES) is obtained in step SK10, and the control flow goes tostep SK12 to determine whether the sum SDN_(SLP) is larger than apredetermined reference SDN_(SLPK). This reference SDN_(SLPK) is anupper limit of the sum SDN_(SLP) above which the engagement force of thelock-up clutch 32 is considered to be excessively small where the dutyratio D_(SLU) is not higher than 40% (where the affirmative decision isobtained in step SK8). For example, the reference SDN_(SLPK) is set tobe in the neighborhood of 100×T1 r.p.m.

If the slip control of the lock-up clutch 32 is normally effected by theslip control means 196, a negative decision (NO) is obtained in stepSK12. In this case, step SK14 is implemented to reset the sum SDN_(SLP)and the time counter CT2, and one cycle of execution of the routine iscompleted. If the sum SDN_(SLP) of the successively obtained controlerror values DN_(SLP) is larger than the upper limit SDN_(SLPK), itmeans that the engagement force of the clutch 32 is excessively smallerthan the value as attained according to the duty ratio D_(SLU)corresponding to the SLIP CONTROL current I_(SLU) generated by the slipcontrol means 196. In this instance, step SK12 is followed by step SK13to determine that there exists the excessive release defect of thelock-up clutch 32. This fact is stored in the transmission controller184. Step SK13 is followed by step SK14. It will be understood thatsteps SK12 and SK13 correspond to the defect detecting means 202.

If the excessive engagement defect or excessive release defect of thelock-up clutch 32 is detected by the defect detecting means 202, thelock-up relay valve 52 is turned OFF and the slip control of the lock-upclutch 32 by the slip control means 196 is terminated.

In the present first embodiment of FIGS. 8B and 10, the slip-parametermonitoring means 198 detects the slip speed N_(SLP) in step SK2 and thecontrol error DN_(SLP) in step SK9, during the slip control of thelock-up clutch 32 by the slip control means 196, and the summing means200 obtains the sum SN_(SLP) in step SK2 and the sum SDN_(SLP) in stepSK9. The defect detecting means 202 determines in step SK5 the presenceof the excessive engagement defect of the clutch 32 on the basis of thesum SN_(SLP) as compared with the predetermined lower limit SN_(SLPK),and in step SK12 the presence of the excessive release defect of theclutch 32 on the basis of the sum SDN_(SLP) as compared with the upperlimit SDN_(SLPK). Thus, the present routine permits reliable detectionof defects of the lock-up clutch 32 associated with the slip control ofthe clutch 32, even when the slip speed N_(SLP) vibrates along awaviness curve, that is, even where the time period during which theslip speed N_(SLP) is held outside a permissible or normal range doesnot exceed a predetermined time.

It is noted that the present embodiment is adapted to effect the firstdiagnosis (SK2-SK7) only when the engagement force of the clutch 32 asdetermined by the duty ratio D_(SLU) is smaller than the predeterminedreference, for example, only when the duty ratio D_(SLU) is higher thana predetermined value (80% or more). Similarly, the second diagnosis(SK9-SK14) is effected only when the engagement force as determined bythe duty ratio D_(SLU) is larger than the predetermined reference, forexample, only when the duty ration D_(SLU) is smaller than apredetermined value (40% or less). In other words, the enabling means206 permits the defect detecting means 202 to operate to effect thefirst diagnosis for the excessive engagement force of the clutch 32 onlywhen the clutch 32 is commanded to have a relatively high slip speedN_(SLP), and to effect the second diagnosis for the excessive release ofthe clutch 32 only when the clutch 32 is commanded to have a relativelylow slip speed N_(SLP). This arrangement is effective to further improvethe diagnosing accuracy and reliability, more specifically, to preventerroneous determination by the defect detecting means 202. In thisrespect, reference is made to the graph of FIG. 11, which shows avariation of the engine speed N_(E) in the case where the target slipspeed TN_(SLP) is continuously reduced to a given value and is then keptat that value. In this case, the engine speed N_(E) is likely to undergoan overshoot, that is, drop far below the desired level determined bythe target slip speed TN_(SLP), as indicated in the graph, even when theamount of slip of the lock-up clutch 32 is normally controlled by theslip control means 196. In this case, the defect detecting means 202would erroneously determine that the engagement force of the clutch 32is excessive with the slip speed N_(SLP) lower than required, if theenabling means 206 or step SK1 were not provided. The present embodimentis free from such erroneous determination of the excessive engagementdefect of the clutch 32.

In the present embodiment, the lock-up relay valve 52 is turned OFF andthe slip control by the slip control means 196 is terminated in theevent of determination of the presence of the excessive engagementdefect or excessive release defect of the clutch 32 by the defectdetecting means 202. This arrangement permits the lock-up clutch 32 tobe fully released even if the spool 136 of the lock-up clutch controlvalve 56 is stuck at a position between its stroke ends corresponding tothe fully engaged and released states of the clutch 32.

Referring next to the flow chart of FIG. 12, there will be described thediagnostic routine executed by the transmission controller 184 accordingto a second embodiment of the invention schematically illustrated inFIG. 8C, for detecting a hunting defect of the slip control of theclutch 32.

The routine of FIG. 12 is initiated with step SL1 corresponding to thespeed-parameter monitoring means 208, to calculate the control errorDN_(SLP) as a speed-related parameter which varies due to hunting in theslip control of the lock-up clutch 32, as indicated in FIG. 13. Step SL1is followed by step SL2 corresponding to the smoothing means 210, tosmooth the control error DN_(SLP) to obtain a smoothed value DNS_(LPSM)(FIG. 13), that is to calculate the smoothed value DN_(SLPSMi) accordingto the following equation (6):

    DN.sub.SLPSMi =DN.sub.SLPSMi-1 +(DN.sub.SLP -DN.sub.SLPSMi-1)/16 (6)

Step SL2 is followed by step SL3 to calculate a difference DDN_(SLP)(FIG. 13) of the actual control error DN_(SLP) with respect to thesmoothed value DN_(SLPSM), according to the following equation (7):

    DDN.sub.SLP =DN.sub.SLP -DN.sub.SLPSM                      (7)

Step SL3 is followed by step SL4 to determine a time period CHGP betweena moment when the difference DDN_(SLP) has increased to a predeterminedlimit tKDNS and a moment when the difference DDN_(SLP) has been reducedto a predetermined limit -tKDNS, and the following time period CHGMuntil the difference DDN_(SLP) has increased again to the limit tKDNS,as indicated in the graph of FIG. 13. Step SL4 is followed by step SL5to determine whether each of the time periods CHGP and CHGM is within apredetermined reference range between tKHG1 and tKHG2. Each of thesetime periods CHGP and CHGM corresponds to a half of the period ofhunting of the control error DN_(SLP).

If a negative decision (NO) is obtained in step SL5, the control flowgoes to step SL6 to reset a time counter CT3 to "0". If an affirmativedecision (YES) is obtained in step SL5, the control flow goes to stepSL7 to determine whether the content of the time counter CT3 is equal toor larger than a predetermined value T3. If a negative decision (NO) isobtained in step SL7, step SL9 is implemented to increment the timecounter CT3. With the routine of FIG. 12 repeatedly executed, anaffirmative decision (YES) is obtained in step SL7, and the control flowgoes to step SL8 to determine that there exists a hunting defect in theslip control of the lock-up clutch 32 by the slip control means 196.That is, the hunting defect is detected if the affirmative decision(YES) is obtained in step SL5 for a predetermined time corresponding tothe predetermined value T3. In this case, the lock-up relay valve 52 isturned OFF to terminate the slip control by the slip control means 196,and the lock-up clutch 32 is brought to its fully released state.

It will be understood that steps SL3-SL5 and SL7-SL9 correspond to thehunting detecting means 212.

In the present second embodiment of FIGS. 8C and 12, the control errorDN_(SLP) (=ΔE) during the slip control of the lock-up clutch 32 by theslip control means 196 is detected in step SL1 by the speed-parametermonitoring means 208, as a parameter which varies due to hunting of theslip control of the lock-up clutch 32. The control error DN_(SLP) issmoothed in step SL2 by the smoothing means 210 to obtain the smoothedvalue DN_(SLPSM). On the basis of the difference DDN_(SLP) between thecontrol error DN_(SLP) and the smoothed value DN_(SLPSM), the huntingdetecting means 212 determines the presence of the hunting defect in theslip control of the clutch 32, by implementing steps SL3-SL5 andSL7-SL9. Thus, the present embodiment permits reliable detection of thehunting defect even where the time period during which the slip speedN_(SLP) is held outside a permissible or normal range does not exceed apredetermined time.

Referring next to the flow chart of FIG. 14, there will be described thediagnostic routine according to a third embodiment of this inventionschematically illustrated in FIG. 8D, for detecting the hunting defecton the basis of a variation of the engine speed N_(E).

The routine of FIG. 14 is initiated with step SM1 to read in the speedN_(E) of the engine 10, speed N_(T) of the turbine impeller 22 andtarget slip speed TN_(SLP). Step SM1 is followed by step SM2corresponding to the engine speed variation determining means 214, todetermine whether the amplitude of a variation of the engine speed N_(E)is larger than a predetermined reference. If an affirmative decision(YES) is obtained in step SM2, the control flow goes to step SM3corresponding to the turbine speed variation determining means 216, todetermine whether the amplitude of a variation of the speed N_(T)+TN_(SLP) is larger than a predetermined reference. If a negativedecision (NO) is obtained in step SM3, that is, if the variation of theengine speed N_(E) is excessive while the variation of the speed N_(T)+TN_(SLP) is not excessive, the control flow goes to step SM4corresponding to the hunting detecting means 218, to determine thepresence of the hunting defect in the slip control of the lock-up clutch32. In this case, the lock-up relay valve 52 is turned OFF to terminatethe slip control of the clutch 32, and the clutch 32 is brought to thefully released state.

The determination in step SM2 may be made by comparing the engine speedN_(E) or the speed (N_(T) +TN_(SLP)) with a smoothed value thereof.

In the present third embodiment, the hunting defect is detected if thevariation of the engine speed N_(E) is excessively large while thevariation of the sum of the turbine impeller speed N_(T) and the targetslip speed TN_(SLP) is not excessively large. This arrangement iseffective to prevent erroneous determination of the hunting detect,which would be made in the case of an instantaneous variation in theturbine impeller speed N_(T) due to a variation in the speed of thedrive wheels. In this respect, it is noted that the hunting in the slipcontrol of the lock-up clutch 32 involves only a periodic variation inthe engine speed N_(E), but substantially no variation in the turbineimpeller speed N_(T), as indicated in the graph of FIG. 15.

The present embodiment is adapted to determine whether the amplitude ofthe variation of the speed N_(T) +TN_(SLP) is larger than the referencevalue, and the hunting defect is not detected if the variation of thespeed N_(T) +TN_(SLP) is larger than the reference even if the variationof the engine speed N_(E) is excessive. This arrangement is effective toprevent erroneous determination of the hunting defect, which would bemade due to a change or updating of the target slip speed TN_(SLP)depending upon the vehicle running condition.

While the present invention has been described above in detail in itspresently preferred embodiments by reference to the accompanyingdrawings, it is to be understood that the invention is not limited tothe details of the illustrated embodiments, but may be otherwiseembodied.

In the first embodiment of FIGS. 8B and 10, the first and seconddiagnoses to detect the excessive engagement and excessive releasedefects are permitted to be effected only while the duty ratio D_(SLU)are held within the respective predetermined ranges (80-100%, and0-40%), namely, only when the affirmative decision (YES) is obtained insteps SK1 and SK8, respectively. However, these steps SK1 and SK8 arenot essential, or the enabling means 206 is not essential.

While the sums SN_(SLP) and SDN_(SLP) used in the first and seconddiagnoses are obtained during the different time periods correspondingto the predetermined count values T1 and T2 of the time counters CT1 andCT2, these time periods may be the same period. Further, the sumSN_(SLP) may be used in both of the first and second diagnoses. In thiscase, the absolute value of the sum SN_(SLP) is compared with a commonreference in step SK5 and SK12.

The routine of FIG. 10 may be modified such that the sum SDN_(SLP) ofthe control values DN_(SLP) is used in the first diagnosis (step SK5) todetect the excessive release defect of the clutch 32 while the sumSN_(SLP) of the slip speed values N_(SLP) is used in the seconddiagnosis (step SK12) to detect the excessive engagement defect of theclutch 32.

Although the second embodiment of FIGS. 8C and 12 uses the control errorDN_(SLP) and its smoothed value DN_(SLPSM), the second embodiment may bemodified to use the engine speed N_(E) and its smoothed value N_(ESM),or the slip speed NSLP and its smoothed value N_(SLPSM).

In the third embodiment of FIGS. 8D and 14, step SM3 is provided todetermine whether the amplitude of variation of the sum N_(T) +TN_(SLP)is larger than a predetermined reference. This step SM3 may be modifiedto determine whether the amplitude of variation of the turbine impellerspeed N_(T) is larger than a predetermined reference.

In the third embodiment, the output of the turbine speed sensor 178 isdirectly used by the turbine speed variation determining means 216 todetect the speed N_(T) of the turbine impeller 22 in step SM1 or toobtain the sum N_(T) +TN_(SLP) in step SM3. The turbine impeller speedN_(T) is equal to the speed Nin of the input shaft 20 of the automatictransmission 14. However, the turbine impeller speed N_(T) may becalculated by using other speed sensors such as the vehicle speed sensor168 adapted to detect the speed Nout of the output shaft 40 of thetransmission 14, or a wheel speed sensor adapted to detect the rotatingspeed of a vehicle wheel. Where the vehicle speed sensor 168 is used,the turbine impeller speed N_(T) may be calculated by multiplying theoutput shaft speed Nout of the transmission 14 by a currently selectedspeed ratio of the transmission 14. Where the wheel speed sensor isused, the turbine impeller speed N_(T) may be calculated by multiplyingthe wheel speed by the speed ratio of the transmission 14 and the speedreduction ratio of the final gear device. In these cases where theturbine impeller speed N_(T) is calculated from the speed Nout or wheelspeed, the turbine speed sensor 178 may be eliminated.

It is to be understood that the present invention may be embodied withvarious other changes, modifications and improvements, which may occurto those skilled in the art, without departing from the spirit and scopeof the invention defined in the following claims.

What is claimed is:
 1. An apparatus including slip control means forcontrolling an amount of slip of a lock-up clutch disposed between apump impeller and a turbine impeller in a power transmission system of amotor vehicle, such that an actual slip speed of said lock-up clutchcoincides with a target slip speed, said apparatus comprising:enginespeed variation determining means for determining whether an amplitudeof variation of a speed of an engine of the vehicle connected to saidpump impeller is larger than a predetermined first reference; turbinespeed variation determining means for determining whether an amplitudeof variation of a speed of said turbine impeller is larger than apredetermined second reference; and hunting detecting means fordetermining the presence of a hunting defect in slip control of saidlock-up clutch by said slip control means, if said engine speedvariation determining means determines that said amplitude of variationof the speed of said engine is larger than said predetermined firstreference while said turbine speed variation determining meansdetermines that said amplitude of variation of the speed of said turbineimpeller is not larger than said predetermined second reference.
 2. Anapparatus according to claim 1, wherein said turbine speed variationdetermining means comprises turbine speed obtaining means for obtainingthe speed of said turbine impeller, slip speed obtaining means forobtaining said actual slip speed of said lock-up clutch, and variationdetermining means for determining whether an amplitude of variation of asum of said speed of said turbine impeller and said actual slip speed islarger than a predetermined value.